One type of ink-jet printer employs ink that is solid under ambient conditions and heated to a liquid state during the printing operation. The solid ink is stored in a reservoir that has a print head mounted to it. The print head includes a firing chamber through which the liquified ink is directed for ejection through adjacent orifices in the print head. The mechanism for ejecting the liquified ink may employ, for example, a piezoelectric element that is responsive to a control signal for abruptly compressing a volume of the liquified ink in the firing chamber thereby to produce a pressure wave that forces the ink drops through the print head orifices.
Typically, solid inks must be heated to approximately 130.degree. C. to reach the liquified state for printing. The resultant temperature increase in the reservoir leads to significant expansion of the volume of air in the reservoir. Further, solid inks normally include volatile jetting agents that contribute to a substantial increase in vapor pressure within the reservoir as the ink is melted. If the fluid pressure increase attributable to the air expansion and vapor pressure increase were permitted to build within the reservoir, the liquified ink would be uncontrollably forced by the high reservoir pressure through the print head. The problem of liquid ink moving in such a way through the print head is known as drooling.
Irrespective of whether there is a substantial increase in fluid pressure within the reservoir, it is typically desirable to establish a slight back pressure within the reservoir so that the liquified ink will remain in the reservoir until deliberately expelled by the activated print head. As used herein, the term "back pressure" means the partial vacuum within the reservoir for resisting the flow of liquified ink through the print head. Back pressure is considered in the positive sense so that an increase in back pressure represents an increase in the partial vacuum. Accordingly, the back pressure is measured in positive terms, such as water column height.
The back pressure in the reservoir must not be so strong, however, that the print head is unable to overcome the back pressure to eject ink.
Ink-jet printers that employ liquid inks often use a thermal-type ink ejection mechanism that includes resistors that are selectively heated for vaporizing portions of ink near adjacent orifices. The rapid expansion of the ink vapor forces drops of ink through the orifices.
Liquid-ink type printers are susceptible to the drooling problem mentioned above and, therefore, require the establishment of a controlled level of the back pressure within the reservoir. The back pressure level must be regulated to account for changes in the fluid pressure within the reservoir, such fluid pressure changes being attributable, for example, to changes in ambient temperature or pressure.